In the so-called flushing method, the pigments which (regularly) occur in the aqueous phase and are given a low water content in filter presses (said pigments having the form of so-called cakes after filtering, said cakes still containing up to 75-80% water) are kneaded in large kneading machines with an organic vehicle, wherein 80-85% of the water contained in the cake is removed and the organic phase, namely the vehicle, replaces the water. The removal of the residual water is accomplished under heating in vacuum. Organophilic pigments, i.e. those which are easily dispersed by organic liquids, are particularly suitable for this method; in the case of hydrophilic pigments, it is necessary to use flushing aids (cf. Roempp, Chemielexikon, Fifth Edition, 1962, Column 1716 and U.S. Pat. No. 609,936 and German Pat. No. 390,812).
It is also known to obtain by drying the pigments which occur with very small particle sizes when precipitated in the aqueous phase. However, this involves a considerable grain enlargement, which must be eliminated once again by subsequent very costly dry milling or milling of the pigment powder in the vehicle, for example in stirring mills, and in many cases the process is not complete.
For this reason, the flushing method has a considerable advantage since the small grain size of the individual pigment particles is retained when the pigment is precipitated.
In the familiar flushing method, kneading machines with a double kneading trough with kneading tools rotating in them are used. The pigment cakes are fed into the kneading machine. The vehicle is then added with the machine running, and the phase change is carried out by intensive kneading; after a certain treatment time the water is largely separated from the cake, as the vehicle is deposited on the pigment. The separated water collects in the upper part of the trough of the kneading machine and is either removed therefrom by tipping the kneading machine until water can run off, or by sucking off the water from the top. In order to tip out the water, a greater structural expenditure is required for the usual size kneading machine; on the other hand, there will always be a danger of pouring off a portion of the oily, therefore viscous, pigment-vehicle paste together with the water. This danger cannot be eliminated even when the water is sucked off, so that in both methods the separated water is not eliminated completely, in order to keep down the loss of pigment and paste.
For this reason, a relatively large amount of water has to be removed by subsequent heat and/or vacuum treatment.